Quantum information processing uses quantum mechanical properties to extend the capabilities of information processing. For example, security of information transfer may be enhanced, the amount of information encoded in a communication channel may be increased, and the number of operations required to perform certain computations may be reduced. Just as in conventional information processing where information is stored in one or more bits, quantum information is stored in one or more quantum bits, known as “qubits.” A qubit may be implemented physically in any two-state quantum mechanical system, such as photon polarization, electron spin, nuclear spin, or various properties of a superconducting Josephson junction, such as charge, energy, or the direction of a current.
One type of qubit based on the plasma oscillation of a superconducting Josephson junction is a circuit known as a “transmon.” Operations on a transmon, such as quantum state initialization, quantum gate operations and quantum measurements, may be implemented by transmitting and receiving microwave photons with the transmon. Microwave photon detectors are used to measure the photons received from the transmon and it would be desirable to provide the detection with high quantum efficiency and low noise.